Nickel-base superalloys are used as the materials of construction of some of the components of gas turbine engines that are exposed to the most severe and demanding temperatures and environmental conditions in the engines. The turbine blades and vanes are typically formed of such nickel-base superalloys, for example. During service, these components are exposed to temperatures of 2000° F. or more, and also to the corrosive hot combustion gases.
The compositions of the nickel-base superalloys are selected to achieve the required mechanical properties in service. However, the available nickel-base superalloys that have the required mechanical properties are not sufficiently resistant to environmental damage to be used for prolonged service. Environmentally resistant coatings are therefore applied to the surfaces of the articles. The environmentally resistant coatings usually include high-aluminum coatings which oxidize to form an adherent aluminum oxide scale that protects the underlying superalloy against oxidation and corrosion damage. A ceramic layer may be deposited overlying the high-aluminum coating to serve as a thermal insulator.
The environmentally resistant coatings work well in a number of applications. In some instances, however, the coated articles develop weakened regions just below their surfaces after extended exposure to elevated temperatures that are encountered in service. The presence of these weakened regions, known in the art as secondary reaction zones or SRZs, leads to the degradation of the mechanical properties of the coated articles. The SRZs have previously been known and recognized in the art, and various techniques have been employed to avoid them or to minimize their adverse effects. See, for example, U.S. Pat. Nos. 5,334,263; 6,080,246; 6,444,053; and 6,447,932, whose disclosures are incorporated by reference. The formation of SRZs is of particular concern in thin-walled articles such as hollow turbine blades or vanes, because the SRZ can weaken a significant fraction of the total wall thickness and lead to premature failure in those areas.
The various approaches to avoiding the SRZs are operable for specific applications, but they may not be applicable in other cases. SRZs are still found as a result of exposure of some of the coated nickel-base superalloys. There is a need for an approach which avoids or minimizes the formation of SRZs in protected articles made of nickel-base superalloys that are otherwise susceptible to its formation. The present invention fulfills this need, and further provides related advantages.